Schedule II - Condensed Financial Information of Registrant (Balance Sheets (Parent Company)) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 30, 2011
Assets:
Cash and cash equivalents	$ 1,023,710	$ 817,602	$ 950,471	$ 795,285
Fixed maturity securities available for sale at fair value (cost $718,642 and $1,317,058 at December 31, 2019 and 2018, respectively)	14,180,961	13,606,812
Equity securities, at fair value (cost $3,430 in 2019 and $3,430 in 2018)	480,620	279,006
Current federal income taxes	13,398	703
Deferred federal income taxes	0	35,490
Property, furniture and equipment at cost, less accumulated depreciation	422,091	416,372
Other assets	762,743	501,413
Total assets	26,643,428	24,895,977
Liabilities:
Other liabilities	1,244,888	1,005,184
Deferred federal income taxes	17,706	0
Subordinated debentures	1,198,704	907,491
Total liabilities	20,525,086	19,416,179
Stockholders' equity:
Preferred stock	0	0
Common stock	70,535	70,535
Additional paid-in capital	1,056,042	1,039,633
Retained earnings (including accumulated undistributed net income of subsidiaries of $5,564,980 and $5,068,139 at December 31, 2019 and 2018, respectively)	7,932,372	7,558,619
Accumulated other comprehensive loss	(257,299)	(510,470)
Treasury stock, at cost	(2,726,711)	(2,720,466)
Total common stockholders’ equity	6,074,939	5,437,851
Total liabilities and equity	26,643,428	24,895,977
Fixed maturity securities available for sale at cost	13,897,964	13,596,057
Parent Company
Assets:
Cash and cash equivalents	389,801	83,950	$ 45,062		$ 124,803
Fixed maturity securities available for sale at fair value (cost $718,642 and $1,317,058 at December 31, 2019 and 2018, respectively)	723,959	1,307,347
Loans receivable	55,794	51,544
Equity securities, at fair value (cost $3,430 in 2019 and $3,430 in 2018)	3,430	3,430
Investment in subsidiaries	7,623,639	6,786,999
Current federal income taxes	18,857	9,068
Deferred federal income taxes	0	66,995
Property, furniture and equipment at cost, less accumulated depreciation	12,323	13,391
Other assets	13,294	12,340
Total assets	8,841,097	8,335,064
Liabilities:
Due to subsidiaries	107,245	116,125
Other liabilities	118,593	115,562
Deferred federal income taxes	22,846	0
Subordinated debentures	1,198,704	907,491
Senior notes	1,318,770	1,758,035
Total liabilities	2,766,158	2,897,213
Stockholders' equity:
Preferred stock	0	0
Common stock	70,535	70,535
Additional paid-in capital	1,056,042	1,039,633
Retained earnings (including accumulated undistributed net income of subsidiaries of $5,564,980 and $5,068,139 at December 31, 2019 and 2018, respectively)	7,932,372	7,558,619
Accumulated other comprehensive loss	(257,299)	(510,470)
Treasury stock, at cost	(2,726,711)	(2,720,466)
Total common stockholders’ equity	6,074,939	5,437,851
Total liabilities and equity	8,841,097	8,335,064
Fixed maturity securities available for sale at cost	718,642	1,317,058
Equity securities available for sale at cost	3,430	3,430
Accumulated undistributed net income of subsidiaries	$ 5,564,980	$ 5,068,139